This invention relates to reinforced structures for the storage and transportation of gases, especially hydrogen containing gases, at high pressure and to a method of manufacturing same and, more particularly, to a reinforced pressure vessel which is especially well suited for subjection to repeated cyclic pressurization, such as in use as a fuel tank for vehicles. The invention also relates to reinforced structures such as reinforced pipe.
Pressure vessels have been produced in a wide variety of designs. For example, early designs were fabricated from high tensile strength alloy steels, which resulted in a substantial weight per unit of volume of vessel, and were subject to hydrogen embrittlement. Of course, these types of vessels tended to be unwieldy and, therefore, had a limited application for portable use.
With the advent of impact extruded aluminum, pressure vessels were improved to the extent that an approximate thirty percent weight reduction was achieved over the conventional steel pressure vessels, while providing an extremely high resistance to industrial and marine environments as well as to many corrosive gases, albeit with relatively limited size and capacity. After the aluminum pressure vessel became well-established, further improvements involved the over-winding of the circumference of an inner liner with a composite material, such as a high strength filament material in an epoxy resin or the like.
The overwound liner design exhibited an increased capacity by a significant amount, with a relatively small increase in weight. However, one problem that has prevented widespread use of the overwound vessels in general use applications is their lack of high cyclic fatigue performance, which is often below 30,000 to 40,000 cycles, well below the 100,000 cycles required for general use cylinders by the United States Department of Transportation. The deficiency is due largely to the fact that these vessels are often designed with their longitudinal burst strength being different from their radial, or hoop, burst strength, both before and after the vessel is overwound with the composite material. This variance between the longitudinal and radial burst strengths causes stress imbalances throughout the vessels and, when very high cycles of pressurization and depressurization occur during use, these stress imbalances cause premature failure, particularly in the "knuckle" radius of the base and head, which is required for producing a vessel by impact extrusion.
Also, these types of vessels were often designed with the thickness ratio between the walls of the vessels and the composite being relatively low. As a result, the vessel would be completely overwound with the composite material, and the head configuration of the vessel often was toroispherical or ellipsoidal in order to keep the filament material in place on the heads during the winding operation. However, this further compounded the stress distribution since, in these designs, the stress at the juncture between the side wall and the head is at least two to three times greater than that in a hemispherical head configuration.
Some pressure vessels according to the prior art also have a relatively short length compared to the inside diameter of the vessel. This leads to a problem known as the "end effect" in which resistance to cyclic fatigue is relatively low due to the fact that the head and base stiffness is transferred to the side wall of the vessel.
The prior art pressure vessels which employ high strength filament material in a matrix usually employ the filament material in a matrix of an epoxy resin or an ordinary polyester resin having limited elongation. These resins have a typical elongation of 2% to 3%, whereas the elongation of the cylinder material is considerably greater, for example, usually 10%-25% for aluminum, depending on the type of aluminum and its thickness. Furthermore, where these resins comprise a matrix for high strength filament material, an even lower elongation is exhibited for the composite material of resin and filaments. The publication "Aluminum Standards and Data 1979", published by The Aluminum Association, Incorporated, defines "elongation" as "the percentage increase in distance between two gauge marks that results from stressing the specimen in tension to fracture". This difference is important where the pressure vessel is initially pressurized, as in an autofrettage process, to obtain a pre-tension in the walls of the pressure vessel. In the autofrettage process, the diameter and length of the vessel are increased as a result of internally applied pressure. A substantial expansion of the vessel also occurs in normal use when it is filled with gas under pressure. By "substantial expansion" is meant expansion of more than 3%. Although the elongation characteristics of the material of the vessel are sufficient to accommodate such substantial expansion, the elongation characteristics of the currently used resins, which are chosen primarily for their corrosion resistance properties, are not sufficient. Thus, the resin matrix containing the high strength filaments wound around the pressure vessel fractures or cracks because of the difference between the expansion of the cylinder and the elongation of the resin matrix. The cracking allows moisture and dirt to migrate into the matrix and engage the wall of the vessel, where they remain and cause corrosion.
It can be seen that a similar problem exists for pipes which are reinforced with high strength filaments in a resin matrix and then autofrettaged or bent along their lengths to fit various applications. For example, when pipe is assembled in a pipeline, the pipe must conform to the supporting earth and where the pipe goes over hills or through depressions, the pipe must bend to conform. The portion of the pipe wall on the outside of the bend often undergoes substantial expansion. The previously used resins would fail where the pipe is bent.
Furthermore, currently used resins are opaque, so that any defects or corrosion which would have been visible in an unwrapped vessel are not visible. Moreover, defects in the filaments and in the resin itself below the surface are not visible. Of course, such defects or corrosion would also not be visible in a similarly wrapped pipe. In addition, especially in the case of aluminum pressure vessels or pipes, exposure to excessive heat often results in critical weaknesses in the wall which are not at all visible, even where the wall is visible.
In prior art pressure vessels which were reinforced with nonmetallic filaments, a filament material ordinarily used was fiberglass of the "S-2" type, which is a relatively expensive and which often snarls and breaks during winding. In addition, in prior art overwound pressure vessels which include a resinous matrix, a resin which is commonly used is epoxy resin, which is expensive. Furthermore, the epoxy resins used have a high cure temperature (on the order of 350.degree. F.-450.degree. F.) and a long cure time (5-8 hours). Not only do these cure characteristics give rise to special handling problems and slow the manufacturing process, but where an aluminum structure is involved, they tend to weaken the aluminum because the cure temperature of the resin is in a range where the strength of the aluminum is significantly weakened and is close to the annealling temperature (450.degree. F.-600.degree. F.) of the aluminum. Since aluminum is normally aged at 340.degree. F. for about 8 hours, the cure characteristics of these epoxies can also result in over-aging the aluminum, which can radically affect the mechanical properties of the metal. For example, over-aging of the metal can cause it to become brittle and thereby fail prematurely. Moreover, epoxy resins present potential health problems and, thus, require special care.